1. Field of the Invention
The present invention relates to a power transmitter and a wireless power transmission system for transmitting power wirelessly.
2. Description of the Related Art
In a conventional audio device, an audio signal source for generating an audio signal and a speaker for converting the audio signal, which is an electric signal, into vibration of air and reproducing the vibration of air as a sonic wave are connected to each other via a cable, and the audio signal is transmitted to the speaker via the cable. However, it is occasionally desirable not to use the cable which connects the audio signal source and the speaker for various reasons such as the installation environment, convenience, appearance and the like of the audio device. In order to respond to such a demand, there are wireless speaker devices for transmitting an audio signal generated by the audio signal source to the speaker wirelessly using wireless communication.
FIG. 17 shows a general structure of a wireless speaker device. A wireless speaker device 1 includes a transmitter 10 and a receiver 20. The transmitter 10 includes a power supply section 14 for receiving power from an electric outlet or a battery, an audio signal source 11, a wireless modulation section 12, and a transmission antenna 13. The wireless modulation section 12 included in the transmitter 10 generates a carrier wave of a frequency used for wireless communication, and modulates the carrier wave with an audio signal generated by the audio signal source 11. The transmitter 10 transmits the carrier wave modulated with the audio signal to the receiver 20 via the transmission antenna 13.
The receiver 20 includes a power supply section 25, a wireless demodulation section 22, an audio signal amplifier section 23, and a speaker 24. The receiver 20 first detects and demodulates the wireless signal, received by a receiving antenna 21, by means of the wireless demodulation section 22 to extract the audio signal. Then, the receiver 20 amplifies the extracted audio signal until the audio signal acquires a voltage sufficient to drive the speaker 24 by means of the audio signal amplifier section 23, and then inputs the audio signal to the speaker 24.
Use of such a wireless speaker device can dispense with a cable for connecting the audio signal source and the speaker to each other. As a result, the audio device has a higher degree of freedom of installation and also has a better appearance.
However, as shown in FIG. 17, the receiver 20 in the wireless speaker device 1 needs to include the power supply section 25 for supplying power to the wireless demodulation section 22 for detecting and demodulating the received signal and to the audio signal amplifier section 23 for amplifying the demodulated audio signal. Therefore, the receiver 20 needs to be connected to an AC power supply or to have a built-in battery. In order to connect the receiver 20 to an AC power supply, a power supply cable is needed for connecting the outlet on the wall to the receiver. This lowers the freedom of installation of the audio device and also deteriorates the appearance of the audio device. In the case where a battery is built in, there is a problem that each time the battery is consumed, the battery needs to be exchanged or charged.
As a technology for solving these problems, for example, Japanese Laid-Open Patent Publications Nos. 2005-117664 and 2009-153056 disclose a device for transmitting power and an audio signal wirelessly.
Japanese Laid-Open Patent Publication No. 2005-117664 discloses a transmitter which rectifies a part of the audio signal to be transmitted and supplies the part of the audio signal to a wireless modulation section in the transmitter as power. A carrier wave generated by the wireless modulation section is modulated with the remaining part of the audio signal. A receiver supplies, as power, a part of a signal voltage obtained by detection and demodulation to a wireless demodulation section for detecting and demodulating the received wireless signal, and thus drives the wireless demodulation section. Owing to such a structure, the audio signal output from the transmitter can be transmitted to the receiver without use of the cable.
Japanese Laid-Open Patent Publication No. 2009-153056 discloses a wireless speaker device which transmits a carrier wave amplitude-modulated with an audio signal from a transmission antenna included in a transmitter by a resonant magnetic coupling method described below. A receiver obtains an AC voltage and a DC voltage for driving an electrostatic speaker based on an AC voltage generated in a receiving antenna. Owing to such a structure, a wireless speaker device requiring no power supply such as a battery or the like can be provided.
Attempts to transmit power wirelessly have been made for various electric devices as well as for wireless speakers as described above. For example, electric shavers and electric toothbrushes adopting a wireless power transmission method using the electromagnetic induction principle (hereinafter, referred to as the “electromagnetic induction method”) have been developed as commercial products. Such wireless power transmission by the electromagnetic induction method is disclosed in, for example, Japanese Laid-Open Patent Publication No. 2008-206233.
Meanwhile, as a method capable of transmitting power for a longer distance than the electromagnetic induction method and thus capable of further raising the freedom of the installation positions of the power transmitter and the power receiver, a wireless power transmission method using a resonance phenomenon of an electromagnetic field is known. This method called the resonant magnetic coupling method is currently under research and development, and is expected to further expand the applicable range of the wireless power transmission. The wireless power transmission using the resonant magnetic coupling method is disclosed in detail in, for example, United States Laid-Open Patent Publication No. 2010/0141042.
According to the resonant magnetic coupling method, the power transmitter and the power receiver both include a resonator. The resonator in the power transmitter (power transmission-side resonator) and the resonator in the power receiver (power receiving-side resonator) are set to have a matching resonant frequency. Owing to this, power is transmitted from the power transmission-side resonator to the power receiving-side resonator using the resonance phenomenon.
In general, the efficiency of the wireless power transmission is in proportion to kQ, which is the product of a coupling coefficient k between the power transmission-side resonator and the power receiving-side resonator and the quality factor of the resonators. Generally in the resonant magnetic coupling method, resonators having a quality factor on the order of several thousand are used. Owing to this, even when the coupling coefficient k is small, for example, even when the distance between the power transmitter and the power receiver is long or even when the axes of coils of the power transmission-side resonator and the power receiving-side resonator are shifted from each other, a relatively high power transmission efficiency can be maintained.
For transmitting power wirelessly, the power transmitter and the power receiver need to communicate to each other via a wireless communication section in order to, for example, confirm that the power receiver is present, perform authentication between the power transmitter and the power receiver, control the power transmission to be started and terminated, and notify a change in the power transmission efficiency caused by a change in the load or a change in the distance between the power transmitter and the power receiver. Such communication made to control the power transmission is disclosed in, for example, Japanese Laid-Open Patent Publication No. 2008-206233 and United States Laid-Open Patent Publications Nos. 2010/0181961 and 2010/0201310.
As one example, communication made to control the power transmission in a wireless power transmission system for transmitting an audio signal in superposition on the power will be discussed. In this case, the power transmitter transmits a request signal, requesting that the state of the power receiver be checked, from the wireless communication section during the power transmission. The power receiver transmits a response to the request signal from the wireless communication section included in the power receiver. In this step, the power receiver needs to drive the wireless communication section in order to receive the signal transmitted from the power transmitter and transmit the response signal. For this purpose, a part of the audio signal transmitted from the power transmitter is converted into DC power and supplied to the wireless communication section as power. As a result, the power supplied to the speaker is decreased as compared with the case where the wireless communication section is not driven. Namely, there is a problem that the volume of the audio signal reproduced from the speaker is changed depending on whether a request signal is received or not.
This problem occurs in wireless power transmission systems for transmitting an audio signal to a speaker and also in other wireless power transmission systems. For example, even in a system in which power and program information are transmitted wirelessly to a liquid crystal display section mounted on a remote control or the like of a TV receiver from a main body, the power transmitter and the power receiver may communicate to each other. In such a system, while communication is being performed, the power which is transmitted is decreased and there may occur a problem that, for example, the luminance of the liquid crystal display section is lowered. Even in a system for transmitting only power but not transmitting information such as voice, video or the like, there may occur a problem that when the power transmitter and the power receiver communicate to each other to control the transmission, stable power transmission cannot be maintained.